Plane parallel plates and optical wedges are commonly used in a wide variety of optical systems and serve an even wider variety of purposes. Examples of applications utilizing plane parallel plates include protective windows, optical beam splitters, optical beam combiners, anti-aliasing filters, amplitude filters, reflective prisms, reticules, etalons, interferometers, and others. When a plane parallel plate is used in collimated space of an incident light source, the plane parallel plate does not introduce aberration into the light beam upon transmission. However when the light source is not collimated, being either convergent or divergent, aberrations are introduced into the optical beam upon transmission.
If a non-collimated light source is normally incident upon a plane parallel plate, then aberration produced by that plate will be rotationally symmetric. Correction of rotationally symmetric aberration produced by a plane parallel plate is often performed by the inclusion of other rotationally symmetric optical components, such as a lens or series of lenses, into an overall optical system. By appropriate design and placement of additional optical components, additional aberrations introduced into the system act to balance out aberration produced by the plane parallel plate. An example of such a system is a traditional lens-based microscope operating in the presence of a cover glass slide. To correct for aberration introduced by the presence of a cover slide, a microscope objective often has a movable group of lenses included as part of the overall system design. For different axial positions of a movable group of lenses, different amounts of aberrations are generated that can balance out the aberrations introduced by potentially variable, movable, and replaceable cover glass slides. Producing a system with multiple movable components requires potentially complex manufacturing processes, as well as a consistent need for recalibration upon the change or movement of any component in the optical system. The rotationally symmetric components utilized within a microscope mainly correct for spherical aberrations, and hence are mainly useful in systems where the objective is telecentric or nearly telecentric.
When a plane parallel plate is tilted such that a divergent or convergent beam of light is incident at a non-normal angle to its surface, the aberrations produced by that plate upon transmission or reflection are not rotationally symmetric. Consequently, the use of rotationally symmetric lenses or mirrors along a common optical axis cannot be utilized to correct aberrations introduced by a tilted plane parallel plate. Similarly, in the case of an optical wedge where the plate has non-parallel sides (either intentionally or due to fabrication errors), rotational symmetry of the optics is destroyed, even if the plate is not tilted. Accordingly, rotationally symmetric lenses or mirrors along a common optical axis cannot be utilized to correct aberrations causes by optical wedges either.
The effect of a tilted plane parallel plate in the optical train may be visualized with reference to FIGS. 1 and 2. Referring to FIG. 1, there is illustrated a schematic diagram of a standard imaging process where three pencils of optical rays are depicted as traveling from an object plane 110 through a uniform medium, such as air, to an image plane 120. The three pencils of rays depicted have angular offsets from an optical axis of −10°, 0°, and 10° respectively. In the absence of any disturbances, the rays converge to perfect focal point images on the same image plane 120. In contrast, FIG. 2 is illustrates an example wherein the three pencils of rays propagate from the object plane 110 to the image plane 120 through a tilted plane parallel plate 210 having a refractive index different than the background medium. The tilted plane parallel plate 210 introduces non-rotationally symmetric aberration 220 at each of the image locations on the image plane 120. Due to the aberration present, the pencils of rays produced from an object plane are not able to focus at a single image plane, and the blurred images from a point source have asymmetric blur. As there is no symmetry between lower and upper blur, rotationally symmetric optics sharing a common optical axis with the object plane 110 and image plane 120 cannot be utilized to correct this blur either for the on-axis pencil of rays or off-axis pencils of rays.
Correcting for such non-rotationally symmetric aberrations has traditionally required either the use of optical components that are also not rotationally symmetric or the tilting of added rotationally symmetric optical components in such a manner that all components in the system do not share a common optical axis. The manufacture and calibration of optical imaging systems containing multiple components that do not share a common optical axis has a much greater complexity compared to systems where all components are aligned along a common optical axis. In addition to increased manufacturing and calibration difficulty, the most commercially available lenses are spherically symmetric components, and consequently the prices for specialized non-rotationally symmetric optical components are likely more expensive and are more difficult to obtain. The aberration compensation achieved through an introduction of one or more non-rotationally symmetric optical components will be operational for a specifically located and oriented tilted plane parallel plate having a particular thickness and material refractive index. Should any of these location, orientation or plate parameters change, the previously introduced non-rotationally symmetric optical elements no longer correct the aberrations produced by the plate and a difficult recalibration or expensive redesign of the optical system will likely be required.